New requirements for user signatures

Many wikis have requirements for custom user signatures.
The Editing team is seeking your input on a proposal to codify some of these requirements in Wikipedia's software.
This will make it easier to reply to specific comments on talk pages and to use some other tools.

You can find more details about what is being proposed, and why, below:

In order for this feature to work well, the software needs signatures to be "machine-readable", so it can reliably detect users' comments and allow replying to them.

The trouble is, while many wikis already have the necessary signature requirements in place, these requirements are not included in the software itself.
This increases the chances someone will set a signature that breaks the wiki's conventions and potentially, make it more difficult for people to participate in conversations.

This additional consistency in the format of signatures would improve existing features, like "mention" notifications, which are only sent if the signature can be detected in your edit.

The three proposed checks are described in this section. These would be applied to user signatures in Preferences when a user saves a modified signature. Under the Editing team's proposal, existing signatures will *NOT* be affected.

Most importantly, this change would disallow unclosed formatting tags, like <i> or the corresponding wikitext markup, '', without a matching closing tag (in this case, </i> or '', respectively). Signatures containing invalid markup can affect the entire discussion page, when the formatting continues into subsequent comments.

The check would also identify misnested tags, like <b>foo<i>bar</b></i> (either both of the i, or both of the b, tags should be on the outside), and stripped tags, which are closing tags without a corresponding opening tag (the opposite of the "unclosed formatting tag" listed above).

Signatures that contain some less critical problems would also be disallowed, e.g., obsolete HTML tags like <tt>...</tt> and <font>...</font>. While these do not cause immediate issues, doing this would prevent the spread of obsolete code to new wiki pages, which is an annoyance for editors cleaning up Linter errors.

Unclosed formatting tags were already supposed to be prevented by the software, but due to limitations of the current wikitext parser, this worked only in some cases. A more robust solution has become possible thanks to Parsoid.

Various tools don't work correctly when a signature does not contain at least one of the following links: a link to the user's user page, user talk page, or contributions page. For example, "mention" notifications are not sent, and forthcoming DiscussionTools will not allow replying to comments with these invalid signatures. Gadgets and other tools that interact with signatures also may not work as expected.

This requirement has been present for a long time in many Wikimedia wikis' policies,[1] but it has not been enforced by the MediaWiki software.

Some use of subst: markup and tildes would also be disallowed in signatures. Previously, it was possible to use these features to set a signature that would cause a subsequent editor's name to be placed on your comments. All forms of signature forgery have long been banned by policy at the larger wikis, and this type of forgery will now be prevented in software. Simple subst: markup is still allowed.

Any existing signatures that would become invalid under the new rules are still allowed (grandfathered in).
When viewing your preferences, you would see a warning message about this, and if you try to change the signature, the new one must be valid.
But if you don't change it, the old invalid signature will continue to be used when you sign, and you'll be able to change your other preferences without affecting it.

We're looking for feedback as to whether you would like existing invalid signatures to be disallowed.
If invalid signatures are disallowed, the default signature would be inserted when affected users sign their comments, until they correct their personalized signatures.